The parasites responsible for malaria in man display different morphologies in the human host and express different antigens depending on their location in the body. The morphological and antigenic differences of these parasites during their life cycles in man enable different stages of development in the liver and in the blood to be defined: the sporozoite, the infectious form injected by the vector mosquito, transforms rapidly into a schizont in the host's hepatocytes and thereafter infects the erythrocytes. The intrahepatic localization of P.falciparum manifests itself in the expression of a group of antigens specific to this stage of development and which are highly immunogenic under the natural conditions of exposure to the disease. This clinically silent phase is at present the only one against which a very strong, sterilizing immunity can be induced experimentally in man, by injecting irradiated sporozoites capable of entering the hepatocyte and of developing therein but without being able to lead on to the blood stage of the disease. Accordingly, the inventors have concentrated the bulk of their efforts on these two pre-erythrocytic stages. However, these stages are also the most intricate ones to study, and hence the least understood, since it is difficult or even impossible to obtain biological material, the only in vitro study model affords a very low yield and the best animal model remains the chimpanzee, the use of which is limited and expensive.
In order to gain access to the antigens of the pre-erythrocytic stages, the inventors used sera of individuals who had resided for 25 years in a region where the disease is endemic but who were on permanent prophylaxis with chloroquine. These individuals were regularly subjected to infected mosquito bites but did not develop any complete blood infection. Their serum hence contained antibodies directed essentially against the pre-erythrocytic stages, which was verified by immunofluorescence (IF) and western blotting on the 3 stages of the parasite.
The use of these sera for screening a library of genomic DNA of the parasitic clone of P.falciparum, the library being constructed in expression vectors in a phage lambda gt11 (V. Rosario, Science 212, 1981, pp. 1037-1038; and Thaithong et al., Transactions of Royal Society of Tropical Medicine and Hygiene, 1984, 78:242-245), led to the demonstration of polypeptides of the pre-erythrocytic stage, in particular the SALSA (sporozoite liver stage antigen) polypeptides described in EP A-0,407,230 and LSA-1 (liver stage antigen) described in WO 92/13884. The present invention relates to new polypeptide molecules specific to the pre-erythrocytic stage, and to their use as active principle of antimalarial vaccine or in methods of diagnosis of the disease.